What is JMX?
JMX shortcut stands for Java Management Extension. JMX was introduced for easy management for local or remote Java applications. The clue element of this is a MBean. MBeans are registered to MBeanServer which is running on JVM with your application. Through MBean you can communicate with your running application and access its data. You even can run methods remotely! But in our case of monitoring we will use MBeans for accessing data from running microservices.
If you want to read more about JMX you can start with Standard MBeans tutorial on Oracle website.
Below is and image showing whole described situation.